In black-and-white motion picture projection films it is frequently desirable to provide an integral sound track. Both the photographic image and sound track images in the film are silver. The sound track, which can be of variable density or variable area, is read optically by a photocell which detects infrared radiation passing therethrough. The peak sensitivity of these photocells, generally referred to as S-1 photocells, is typically at about 800 nm plus or minus 50 nm. The wide variance in peak absorption is of little importance, since silver has a substantially uniform absorption in the infrared region of the spectrum.
In color photography, instead of employing silver images, as in black-and-white photography, the oxidized developing agent which is generated in imagewise developing silver halide to silver is used to form a dye image. The formation of color photographic images by imagewise reaction (coupling) of oxidized aromatic primary amine developing agents with incorporated color-forming couplers to form dyes is well known. In these processes, the subtractive process of color formation is ordinarily used, and the image dyes customarily formed are cyan, magenta and yellow, the colors that are complementary to the primary colors, red, green and blue, respectively. The silver image which is formed by development is an unwanted by-product which is removed by bleaching.
In color motion picture projection films it is conventional to employ a silver sound track. The rquirement that silver be retained in the optical sound track of the motion picture film is distinctly disadvantageous because the developed silver must be removed from the picture area without disturbing the silver in the optical sound track. This has given rise to processing techniques which require the separate treatment of a portion of the film at least once during processing in order to obtain a silver sound track.
The desirability of employing dye sound tracks in color motion picture projection films, particularly dye sound tracks compatible with projection equipment now in use designed for films having silver sound tracks, has been long recognized. Unfortunately, the subtractive dyes which form the picture image have their regions of maximum absorption in the range of from about 400 to 700 nm and are relatively transparent in the infrared region where the S-1 photocells are most sensitive. In looking for dyes suitable for use in forming infrared absorbing sound tracks for color motion picture projection films two principal obstacles have been encountered. First, the dyes have for the most part lacked sufficient peak absorption in the required region of the spectrum. Second, the absorption peaks of the dyes have not been broad enough to accomodate the plus or minus 50 nm variation in peak sensitivity of S-1 photocells. Infrared absorbing dyes which have been disclosed for use in forming integral dye sound tracks are illustrated by Vittum et al U.S. Pat. No. 2,266,452, issued Dec. 16, 1941, and Frohlich et al U.S. Pat. No. 2,373,821, issued Apr. 17, 1945. More recent disclosures which address maximum absorption peak densities, but which do not address the breadth of the absorption peak, are illustrated by Japanese Publication No. 59838, laid open Aug. 22, 1973, based on patent application No. 94266, filed Nov. 24, 1971, and U.K. Pat. No. 1,424,454.